Stress measuring means



@im fw@ ma? X l l l nu .7. am m A! 3 ..mls.i:z,1941. c'. M. KEARNSJR., Em. 2,252,464k SUBSTI-TUTEOR y STRESS Mmsuam@ MEANS l i I Filed Dec. 6, 1937.,

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Aug.12,1941.. "Q M. KEAR$-JRJ+A 2,252,4@4

' STRESS MEASURING MEANS:

Filed Dc. 6, 1553*( 32 f Mns. V-

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velectrical current.

tory testing or resistance measuring circuit for 'i Patented ug. 12, 19111 PATENT orrlcs y ls'rruass mancanze mus Charles M. Kearns, Xt., East Hertford, Conn., and

Ralph Ei. Goethe, ommereiic, Macs., asuigni ors to United discreti orporetion, East Hartv tord, Conn., a corporation of Delaware .ippncsum nece-mee.- s, rear, sono no. itam s calms. (ci. 1v1-aaa) Y This invention relates to an improved method und apparatus for measuring strains in bodies subjected to stresses and has particular refer-f ence in su improved method and apparatus for measuring strains in bodies subjected to abrupt 5 changes in stress such as bodies subjected to impact loads or vibrations.

An object of the invention resides in the provision of en improved method end apparatus of the character speciied by Ineens of which strinns or variations in strains in a body may be indicated by variations in an electrical current.

further object resides in thel provision of an improved method end apparatus of the character described, by means of which the magnitude of the strain or the magnitude of the variation in strain in abody subjected to test may be measured by the magnitude of an electrical current or by the magnitude of the changes in an A still fru'ther object resides in the provision of en improved method and apparatus of the character indicated which may be utilized to measure the strains or variation in strains in a moving or rotating body.

An additional object resides in the provision of an improved method and apparatus of the character indicated which may be utilized to measure the strain or strain variation in e. rapidly rotating bodyr end which will not be rendered inaccurate by the elect of the centrifugal forces acting on the body.

Other objects and advantages will bfmore particularly pointed out hereinafter or wm become apparent as the description proceeds.

In the accompanying drawings in which like reference numerals are used to designate similar UL the purpose of illustration only and are not to be taken as limiting or restricting the invention. In the drawings, Fig. 1 is a schematic illustration of au apparatus for measuring strains or strain variations in a rotating body such as an aeronautical propeller. Fig. 2 is a schematic illustration showing a somewhat. different arrangement of the apparatus illustrated 'in Fig. l. y

Fig. 3 is a schematic illustration oi an elemendetermining the electrical conductivity charace teristics of certain elements of tue apparatus.-

Figi/l is a schematic illustration of an eiemenn line 1--1 of ne. e.

Fig. 8 is a sectional view on the line 8-8 of Fig. 6. and

Fig. 9 is an approximate reproduction of an oseiilogram produced by the improved method and apparatus.

Referring to the drmvings in detail and particuierly to Fig. 1, the numerel i0 generally indientes an aeronautical propeller which has been 1 sho-wn as an example of a body in which the strain or strain variations are to be measured. It

whether moving In carrying out the improved method certain points ou the hub l2 or blades i6 of the propeller are selected et which it is desired to measure the l strain or strain variations. In the illustrative examples shown, two points on each blade have Y been selected es indicated at a, b. c and d.- At each of these points a strain sensitive device,

hereinafter referred to as an extensometer, is

secured to the surface o! tho blades.

The extensometer and the manner of attaching it to the surface of the Jody in which the strain or strain variation is to be measured is particularly illustrated in Figs. 6, 7, and 8. The

extensometer i6 is a re'iretively liet,` elongated strip' of a. material including carbon particles, distributed among particles of a material such as silica in a proportion of approximately one to se"4 en and held together by a suitable binder hav.

ing an appreciable degree of resiliency such as a resinous or plastic cement or phenolic condensation product. The extensometer i6 may be of any suitable dimensions but one that has been* successfully used has a length of approidmately 1% in., a width of .approximately of an in.

and a thickness or approximately or an inch. t

considerable degree a matter of choice but the particular extensometcr illustrated in Figs. 6, 'I and 8 has c-ne flat suriacejoined to a cylindrical surface and may be formed by grinding flat a cylindrical prism of suitable material. In applying the extensometer to the surface. the surface is first thoroughly cleaned and then coated with a suitable cement as indicated at I8 in Figs. 7 and 8. A thin strip of paper D is then applied to the coated surface and the extensometer i6 is cemented to the outer surface oi the paper as indicated at 22. In Fiss. 7 and 8 the cement and paper layers have been shown as having an exaggerated thickness for the purpose of clearness and simplicity in the illustration. While various kinds of cement might be used for attaching the vextensl'lrneter for the surface to be tested, the cement used should have the quality of being applicable in extremely thin layers and should be suiilciently hard so that the length of the extensometer will be forced to follow substantially exactly the changes in length in the surface to which it is attached. A commercial at their ends to the coated end portions of the extensometer by suitable means such as solder.

Preferably two different kinds of circuits are f provided in connection with the extensometer and means, such as the switches schematically K indicated at 28, 2t and t6, for connecting any one of the extcnsometers into either o! the circuits as may be desired. One ci the circuits is an extensometer testing or resistance measuring circuit, schematically illustrated in elementary form in Fig. Il,v and the other is a strain measuringv circuit, schematically illustrated in elementary form in Fig. 4. In the testing circuit the extensometer I6 is connected into a Wheatstone bridge circuit in series with a variable resistance S2 of known characteristics and in parallel with two fixed resistances S4 and 36 of an order such that the galvcnometer 38 will be balenced when the variable resistance 32 has exactly the same resistance as the extensometer IB. The bridge is connected in the usual manner to the terminal of a battery which supplies the necessary current for measuring the resistance ot the extensometer.

The strain measuring circuit schematically i1- lustrated in Fig. 4 includes an exciter circuit in which the current passes from one terminal of the battery 6 through the extensometer t8 and through the variable resistance 32 in series and out-.1er lead et and me additional muet les@ es'.

alyzer E2, and the ground connection 8l. In this ,Y circuit any variation in the resistance of the extensometer le with respect to the resistance ot the respective resistor 32 will cause a proportional fluctuation in the current passing through the battery circuit described above. These current fluctuations will induce fluctuations o! increased magnitude at the outlet 56 of the ampliiier 50 which amplified current fluctuation may be recorded by the osciilograph 52 or indicated by a suitable current analyzer. The exciter circuit may be coupled to the indicating circuit either cepacitivcly, as indicated above, or inductively by means ci a suitable transformer if desired.

The circuits illustrated in Fig. 1 include an extensometer testing circuit as shown in Fig. 3 and a strain measuring circuit as illustrated in Fig. 4. These two circuits are connected with the extensometers carried at a. b, c, and d upon the blades of the rotating propeller through a set of suitable collector rings 58. The circuit includes a respective variable resistor 22 for each extensorneter and suitable switches 69, 62 and 64 for alternatively connecting the extensorneters with either the testing circuit or the Strain measuring circuit. The switches 26 and 28 are operative to select any particular one of the various extensometcrs for connection into either of the circuits mentioned. As shown in Fig. 1 the amplier 5@ has been illustrated as a. twochannel amplifier so that the indicated strain variation o! two seperate extensometers may be reproduced at the same time. This construction calls for an additional lead 42 and condenser v t? connectable to the extenscrneters through the switch 25. The amplicr is connected to a double element oscillogreph 52 by means of the Obviously two current analyzers may be substituted for the double element oscillograph if fdesired. It will be understood that each of the switches schematically illustrated at 26, 28 and 36 is either a. plurality of independently operable switches or a multi-pole rotatable switch so that onlyone connection need be made at a.

. citer circuit, including the battery, the extensometers and the variable resistors, are carried by the propeller and on rthe same side oi' the I slip rings E8. The indicating circuit only is carried through the slipA rings and the selector switch 26 to the amplifier, thereby eliminating back to the battery, and an indicating circuit 1 energized by current variations in the exciter circuit. When measuring the strain the variable resistance 32 is preferably always so adilzsted that its resistance is exactly the same as the resistance of the extensometer. although a variable or fixed resistance not entirely in balance with the extensometer may be used to give approximate resuits which may be lsuiiciently ac.- curate for'the particular purpose or' :nay be :reduced to a suiilcient accuracy by computation.

The exciter circuit is coupled by means of leads E and It through the respective eender-.sera 4I and 3 i i the indicating circuit including the amfrom the stress readings obtained substantially. all of the possible error incident to variations in the conductivity of the slip rings and associated brushes. y

Fig. 9 shows an approximate reproduction of a record obtained from an oscillograph when measuring vibrational strains in a rotating propelicr by the improved method hereinabove set lcrth.' The amplitude ot the wave form of the curves shown in Fig.' 9 give a comparative' indication o! the magnitude of the strains atthe points at which the measurements were taken. This comparative indication can readily be reduced toa quantitative measurement of strain it the characteristics of the extcnsometers are-r dehnitely known. The n invention includes a.V v plier 5G, the oscillograpb. or other current an- 75 method and apparatus I or denitely determining f .n..ww-m.. ....i

in quantitative values the proportionate strainresistance characteristics of the exiensometers.

In order to calibrate the extensometers for quantitative results one or more extensometers to be calibrated are secured upon a test beam. indicated at 58 in Fig. 5, in the mannerillustrated in Figs. 6, 7 and 8 and described in connection therewith. The beam 58 is lsecurely clamped at one end as indicated at 60 and, for test purposes is loaded at'the opposite end as indicated at 62.y

Preferably, the beam 58 is of the same material as the bodies to be tested so that additional computations to correct for diierences in the modulus of elasticity of the two bodies will not ce necessary. The opposite ends of the extensometcr I6 are connected into a Wheatstone bridge circuit including twoxed resistances 64 and 68 and a valuable resistor of known characteristics 68 so that the resistance of the extensometer at any deflection of the beam 58 can be accurately determined in terms of ohms resistance. same time the strain in the beam 58 is measured by some suitable form of direct reading strain gauge such'as the Huggenberger strain gauge indicated at '10. By this method a definite determination of the resistance in ohms o1' the extensometer incident to the strain in the beam upon which itis mounted. can be accurately determined. With the increment of increased resistance incident to a unit increased in strain detinitely known for each extensometer, the current fluctuation caused by the variations in resistance of the extensometers mounted upon a body being tested can be easily converted into units of strain in the test body.

I! a number of extensometers are calibrated at the same time by the apparatus illustrated in Fig. 5, a suitable switch will be provided between cor responding ends of the extensometers and the `point of attachment of those ends into the W'heatstone bridge circuit so that the extensometers may be selectively connected into the circuit.

'With the extensometers of determined characteristlcs the method illustrated in Fig. 5 may Having now describedthe invention so that Y others skilled in the art may clearly understand the same what it is desired to secure by Letters Patent is as follows:

l. A resistance element [or an electrical strain gauge comprising, a thin elongated element of a substantially uniform mixture of iinely divided carbon and silica and a resilient binder having an elasticityl similar to that of phenolic resin.

n shaped to conform substantially to the surfaceA At the of a test body.

3. A resistance element for an electrical strain gauge comprising, a. thin elongated element of a substantially uniform mixture of iii-.ely divided carbon and silica and a resilient binder having an elasticity similar to that of phenolic resin, one side, of said element being flattenedfor attachment of said element to a test body.

4. A resistance element for an electrical strain gauge and a mounting therefor comprising, a thin elongated element of a substantially uniform mixture of finely divided carbon and silica and a resilient binder, said element having at least one side shaped to conform approximately to the surface of a test body, a strip of tliin fibrous material cemented to the surface of said test body, and a layer of resilient cement between and connecting said fibrous material and said surface of said element.

5. A strain sensitive electrical resistance ele-v ment for use in electrical strain indicating apparatus comprising: a tia.: elongated strip of a composition including carbon particles and particles of an inert material intermingled and held together by a suitable binder with an elasticity having a value similar to that of phenolic resin.

6. A strain sensitive electrical resistance eiement for use in an electrical apparatus for measuring strains comprising, a thin elongated strip of small dimensions and light Weight of a composition including intermingled carbon and silica particles in a ratio of approximately one to seven, l

I in a binder having a. degree of resiliency similar connection with Figs. 6, 7 and 8 to the body when Y in an unstressed condition. The body may then be loaded in any desired manner and the incident strain measured in terms of variations in the resistance of the extensometer and converted into units of strain through the quantitative data obtained by Calibrating the extensometers.

While one manner of applying the improvedy to that of phenolic resin.

7. An electrical resistance element for a strain y indicator and a mounting therefor comprising a electrical strain gauge or vibration analyzer comprising, a relatively thin elongated strip of materia! including carbon particles distributed ina. i

mass of silica particles in a. ratio of approximately one to seven and held in position by a.A

resilient binder, said strip being provided with metal coated end portions for the attachment of lead wires thereto.

CHARLES M. KEARNs, Jin.. Y RALPH M. GUERRE. 

